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TOKYO — Aguri Suzuki, a 44-year-old real estate agent, said she sometimes thinks the ground is shaking even when it is not. When she sees a tree branch swaying in the wind, she worries there has been an earthquake.

Doctors here say they are seeing more people who are experiencing such phantom quakes, as well as other symptoms of “earthquake sickness’’ such as dizziness and anxiety.

And it is no wonder. As if the threat of radiation from a crippled nuclear power plant were not enough, Tokyo and the region to its northeast have been under a constant barrage of aftershocks since the magnitude 9.0 earthquake that set off a devastating tsunami on March 11. Two earthquakes were felt in Tokyo yesterday morning, three on Tuesday, a large one on Monday, and a very large one of 7.1 magnitude last Thursday.

Overall, there have been 400 aftershocks of 5.0 magnitude or greater in northeastern Japan since March 11. That is as many sizable quakes in one month as Japan typically experiences in two and a half years, the Japan Meteorological Agency said.

Geologists said the frequency of aftershocks has declined since March 11 and will continue to fall, but they will remain higher than normal for a long time.

The quakes are complicating efforts to control the Fukushima Daiichi nuclear power plant. For instance, the quake on Monday knocked out cooling at the Fukushima plant for nearly an hour.

Every time a sizable quake occurs, the first question on many people’s minds is whether the nuclear plant has been further damaged and whether a new cloud of radiation is on the way.

Government officials are becoming concerned that in the rush to cool the reactors and prevent hydrogen explosions, the plant’s vulnerability to another tsunami has been overlooked.

“A week ago we thought the major risk was a hydrogen explosion,’’ a senior official in the office of the prime minister said Tuesday. “I think the major risk at the moment is an aftershock and tsunami.’’